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Brachiosaurus is a genus of sauropod dinosaur from the Jurassic Morrison Formation of North America. It was first described by Elmer S. Riggs in 1903 from fossils found in the Grand River Canyon (now Colorado River) of western Colorado, in the United States. Riggs named the dinosaur Brachiosaurus altithorax, declaring it "the largest known dinosaur". Brachiosaurus had a disproportionately long neck, small skull, and large overall size, all of which are typical for sauropods. However, the proportions of Brachiosaurus are unlike most sauropods: the forelimbs were longer than the hindlimbs, which resulted in a steeply inclined trunk, and its tail was shorter in proportion to its neck than other sauropods of the Jurassic.

Brachiosaurus is the namesake genus of the family Brachiosauridae, which includes a handful of other similar sauropods. Much of what is known by laypeople about Brachiosaurus is in fact based on Giraffatitan brancai, a species of brachiosaurid dinosaur from the Tendaguru Formation of Tanzania that was originally described by German paleontologist Werner Janensch as a species of Brachiosaurus. Recent research shows that the differences between the type species of Brachiosaurus and the Tendaguru material are significant enough that the African material should be placed in a separate genus. Several other potential species of Brachiosaurus have been described from Africa and Europe, but none of them are thought to belong to Brachiosaurus at this time.

Brachiosaurus is one of the rarer sauropods of the Morrison Formation. The type specimen of B. altithorax is still the most complete specimen, and only a relative handful of other specimens are thought to belong to the genus. It is regarded as a high browser, probably cropping or nipping vegetation as high as possibly 9 metres (30 ft) off of the ground. Unlike other sauropods, and its depiction in the film Jurassic Park, it was unsuited for rearing on its hindlimbs. It has been used as an example of a dinosaur that was most likely ectothermic because of its large size and the corresponding need for forage, but more recent research finds it to have been warm-blooded.